A magnetic disk device, commonly termed a "disk drive", is a storage device used in data processing system for storing retrievable digital data in magnetic form. The data is stored on a rotating magnetic disk in a set of concentric circular patterns termed "tracks". A read/write head is mounted on a carriage that moves the head radially to bring it to a desired track and then maintains it in position over that track so that the head can record a series of data bits or, alternatively, retrieve a series of bits from the track as the latter rotates under the head. Large-capacity disk drives incorporate a plurality of such disks mounted for rotation together on a single spindle. A least one separate read/write head is used for each disk surface, all the heads being mounted on the same carriage to provide a comb-like arrangement in which the heads move in and out in unison.
The carriage on which the read/write heads are mounted is incorporated in a servo system which performs two distinguishable functions in moving the carriage. The first of these is a "seek" or "access" function in which the servo system moves a read/write head to a selected track from a previous track which may be a substantial number of tracks distant. When the head reaches the desired track, the servo system commences a "track following" function in which it accurately positions the head over the centerline of the selected track and maintains it in that position as successive portions of the track pass by the head.
The seek and track-following functions impose different constraints on the servo system. During a seek operation the carriage must be moved as fast as possible so as to minimize the time required for that operation. Velocity accuracy is also important in establishing a velocity trajectory and good arrival characteristics. During a track-following operation, on the other hand, position accuracy is a most important factor. The accuracy with which the read/write head can be made to follow the track centerline is a determining factor for the track density on the disk. That is, the closer the head can be made to follow track centerlines, the closer together can the tracks be spaced.
The head-positioning servo system senses the position of the read/write head by means of servo signals recorded in tracks on the disk pack. In one conventional arrangement the servo signals are recorded on a dedicated servo surface, i.e., a surface which contains only these signals. In another conventional arrangement the servo signals are embedded in the data. That is, they are recorded in servo fields at the beginnings of the data track sectors. The embedded servo signals have the capability of providing more accurate data head position information than the dedicated servo signals. However, because they are spaced apart by the data sectors on the data tracks and are thus sampled periodically at a relatively low rate, they are incapable of providing position signals having high frequency components. On a dedicated servo surface, on the other hand, the servo signals on each track are essentially continuous and thus they can provide position information having a substantially broader frequency band.